THE EVOLUTION OF PLANTS 



orchids, etc.). This, in bare outline, is the monophyletic 

 hypothesis (Fig. 11). 



Other students, on what they believe to be equally good 

 evidence, postulate two primitive main branches, appearing 

 as distinct at the dawn of the fossil record — the club-moss 

 stock, or Lycopsida, and the fern stock, or Pteropsida, from 

 which have descended the modern conifers and flowering 

 plants and their ancestors. This is one of the polyphyletic 

 hypotheses (Fig. 13). 



The solution of the question of the "family-tree" of plant 

 life may be roughly likened to the task of putting together a 

 picture-puzzle, in which many of the pieces are not under- 

 stood and some are perhaps temporarily or permanently lost. 

 If we had a museum collection of specimens of all the kinds 

 of plants that have ever lived, botanists believe that such 

 specimens could be so arranged as to represent their genetic 

 relations and to give us a true picture of the evolutionary 

 development of the present plant world. But probably no 

 such collection can ever be made. We are continually find- 

 ing, with more or less certainty, where this or that piece 

 belongs in the picture, and lost pieces are continually being 

 discovered as fossils in the rocks or as facts disclosed in the 

 laboratory and field. 



Again — to use once more the illustration afforded by the 

 picture puzzle — it is the difficulty of the problem that fasci- 

 nates the scientist, and it is the modicum of his success that 

 lures him on to further research; he finds his reward in the 

 quest and in the satisfaction of making some contribution, 

 however slight, to the ultimate, but probably unattainable 

 success. 



It is one thing, however, to accept evolution as a fact and 

 quite another thing to explain the method of evolution — how 

 this gradual change or series of changes has been brought 



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